Solid fuels have a number of significant advantages before fuel oil; they are generally cheaper, they are available in large amounts, and they take part in a natural circulation and do not cause pollution load on the environment in spite of their emission of carbon dioxide, when they are based on wood or other renewable bio-products. Nevertheless, solid fuels are used to a comparatively small degree in the modern society. The main reason for this condition probably is that it is easy to automatize combustion of fuel oil but comparatively difficult to automatize combustion of solid fuel, and it is particularly difficult to automatize solid fuel combustion in order to provide an efficient combustion at all heating effect levels without emission of products with the fuel gases which are harmful to the environment.